You can't log in to your account with your username.

You're unable to select a desired username.

Some usernames may be unavailable for use because they are taken by other users or are unavailable for other reasons. Tips for selecting an available username and checking to see what's available are here.

Trying to change your username on a mobile device? We're sorry, but the ability to change your username is not currently supported on Twitter's mobile apps. You'll need to log in via twitter.com or mobile.twitter.com in order to change your username. Click here for instructions on how to do that.

A username is in use by another Twitter account that you own.

If you're trying to move a username from one of your accounts to another of your accounts, you can do this yourself.

Log into the account that currently has the desired username.

Change the username to something else (try adding a number or underscore). This will immediately free up the username for use by other accounts.